/*
Copyright (c) 2010-2013 Thomas Peri
http://www.tumuski.com/

Permission is hereby granted, free of charge, to any person obtaining a
copy of this software and associated documentation files (the
"Software"), to deal in the Software without restriction, including
without limitation the rights to use, copy, modify, merge, publish,
distribute, sublicense, and/or sell copies of the Software, and to
permit persons to whom the Software is furnished to do so, subject to
the following conditions:

The above copyright notice and this permission notice shall be included
in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY
CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT,
TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE
SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
*/

/*jslint white: true, browser: true, onevar: true, undef: true, nomen: true,
	eqeqeq: true, plusplus: true, regexp: true, newcap: true, immed: true */
// (good parts minus bitwise and strict, plus white.)

/**
 * Nibbler - Multi-Base Encoder
 *
 * version 2013-04-24
 *
 * Options:
 *   dataBits: The number of bits in each character of unencoded data.
 *   codeBits: The number of bits in each character of encoded data.
 *   keyString: The characters that correspond to each value when encoded.
 *   pad (optional): The character to pad the end of encoded output.
 *   arrayData (optional): If truthy, unencoded data is an array instead of a string.
 *
 * Example: 
 *
 * var base64_8bit = new Nibbler({
 *     dataBits: 8,
 *     codeBits: 6,
 *     keyString: 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/',
 *     pad: '='
 * });
 * base64_8bit.encode("Hello, World!");  // returns "SGVsbG8sIFdvcmxkIQ=="
 * base64_8bit.decode("SGVsbG8sIFdvcmxkIQ==");  // returns "Hello, World!"
 *
 * var base64_7bit = new Nibbler({
 *     dataBits: 7,
 *     codeBits: 6,
 *     keyString: 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/',
 *     pad: '='
 * });
 * base64_7bit.encode("Hello, World!");  // returns "kZdmzesQV9/LZkQg=="
 * base64_7bit.decode("kZdmzesQV9/LZkQg==");  // returns "Hello, World!"
 *
 */
var Nibbler = function (options) {
	"use strict";
	
	// Code quality tools like jshint warn about bitwise operators,
	// because they're easily confused with other more common operators,
	// and because they're often misused for doing arithmetic.  Nibbler uses
	// them properly, though, for moving individual bits, so turn off the warning.
	/*jshint bitwise:false */
	
	var construct,
		
		// options
		pad, dataBits, codeBits, keyString, arrayData,
		
		// private instance variables
		mask, group, max,
		
		// private methods
		gcd, translate,
		
		// public methods
		encode, decode;
	
	// pseudo-constructor
	construct = function () {
		var i, mag, prev;

		// options
		pad = options.pad || '';
		dataBits = options.dataBits;
		codeBits = options.codeBits;
		keyString = options.keyString;
		arrayData = options.arrayData;
		
		// bitmasks
		mag = Math.max(dataBits, codeBits);
		prev = 0;
		mask = [];
		for (i = 0; i < mag; i += 1) {
			mask.push(prev);
			prev += prev + 1;
		}
		max = prev;
		
		// ouput code characters in multiples of this number
		group = dataBits / gcd(dataBits, codeBits);
	};

	// greatest common divisor
	gcd = function (a, b) {
		var t;
		while (b !== 0) {
			t = b;
			b = a % b;
			a = t;
		}
		return a;
	};

	// the re-coder
	translate = function (input, bitsIn, bitsOut, decoding) {
		var i, len, chr, byteIn,
			buffer, size, output,
			write;
		
		// append a byte to the output
		write = function (n) {
			if (!decoding) {
				output.push(keyString.charAt(n));
			} else if (arrayData) {
				output.push(n);
			} else {
				output.push(String.fromCharCode(n));
			}
		};

		buffer = 0;
		size = 0;
		output = [];
		
		len = input.length;
		for (i = 0; i < len; i += 1) {
			// the new size the buffer will be after adding these bits
			size += bitsIn;
			
			// read a character
			if (decoding) {
				// decode it
				chr = input.charAt(i);
				byteIn = keyString.indexOf(chr);
				if (chr === pad) {
					break;
				} else if (byteIn < 0) {
					throw 'the character "' + chr + '" is not a member of ' + keyString;
				}
			} else {
				if (arrayData) {
					byteIn = input[i];
				} else {
					byteIn = input.charCodeAt(i);
				}
				if ((byteIn | max) !== max) {
					throw byteIn + " is outside the range 0-" + max;
				}
			}
			
			// shift the buffer to the left and add the new bits
			buffer = (buffer << bitsIn) | byteIn;
			
			// as long as there's enough in the buffer for another output...
			while (size >= bitsOut) {
				// the new size the buffer will be after an output
				size -= bitsOut;
				
				// output the part that lies to the left of that number of bits
				// by shifting the them to the right
				write(buffer >> size);
				
				// remove the bits we wrote from the buffer
				// by applying a mask with the new size
				buffer &= mask[size];
			}
		}

		// If we're encoding and there's input left over, pad the output.
		// Otherwise, leave the extra bits off, 'cause they themselves are padding
		if (!decoding && size > 0) {
		
			// flush the buffer
			write(buffer << (bitsOut - size));

			// add padding string for the remainder of the group
			while (output.length % group > 0) {
				output.push(pad);
			}
		}

		// string!
		return (arrayData && decoding) ? output : output.join('');
	};
	
	/**
	 * Encode.  Input and output are strings.
	 */
	encode = function (input) {
		return translate(input, dataBits, codeBits, false);
	};
	
	/**
	 * Decode.  Input and output are strings.
	 */
	decode = function (input) {
		return translate(input, codeBits, dataBits, true);
	};
	
	this.encode = encode;
	this.decode = decode;
	construct();
};

module.exports = Nibbler;
